I work in a restaurant. I owned one for a while, too. I can tell you, there are those that want the stuff.
The company I had taking mine away, at first, charged me to do so. Another company approached and said they'd charge less, they got into a bidding war with each other. Ended up having it done for free. At the time, I believe it was going into C4 production.
Now, at the restaurant I work at, the gentlemen who delivers our mushrooms takes it, and actually PAYS us for it (in the form of an extra box of mushrooms; product we'd use anyway). They use it to keep their 'cave' system at ideal temps, we get free product and free removal of something we have no use for. Win/Win.